It is known to attach a plastic component such as a bracket to a plastic base member such as an automotive instrument panel by the vibration welding. As the vibration welding allows the component to be attached to any part of the base member, it is necessary to provide a means for positioning the component to the base member at the time of welding. In the welding arrangement disclosed in Japanese patent laid open publication No. 2004-90697, projections that can resiliently deform in a prescribed direction project from an instrument panel of a motor vehicle, and mating grooves corresponding to the projections are formed in the component so that the two parts may be properly positioned relative to each other by fitting the projections into the corresponding mating grooves and vibration welding the two parts to each other by oscillating the component in the prescribed direction by using a vibration welder.
However, according to this proposal, during the welding process, the projections inevitably deflect to a certain extent as a result of the relative movement caused by the welding process, and the component may shift in position relative to the instrument panel during the welding process. When a high positional precision is required, such a positional shifting is not tolerable.
Also, when a large number of component parts are required to be attached, a correspondingly large number of projections are required to be formed on the side of the instrument panel. However, because the instrument panel is a relatively large plastic component, it is not desirable to complicate the shape of the instrument panel in view of minimizing the manufacturing cost.